Cross Country wheels are made to be fast and this makes it necessary to reduce resistance where possible. On a rough surface, rolling resistance can be up to 69% of total resistance. 

Rolling resistance is defined as: rolling resistance coefficient x weight x gravitational acceleration. Whereas the coefficient is defined by the wheel size; tire section; tire characteristics, tire pressure, tread, soil.

While most of those factors cannot be influenced by the wheel, we can modify the tire support.
A wider rim provides improved tire support which allows lower pressure and leads to less rolling resistance on rough tracks.

During cornering, lateral forces occur which act on the tire. The tire’s contact patch moves in the opposite direction to the lateral force. 
When riding through a corner with a narrow rim, the tire’s contact patch is no longer above the flange of the narrow rim. The tire is no longer supported by the rim and folds, resulting in an unstable riding experience. This folding creates tension over the tire which pulls on the opposite bead and causes a loss of air between the tire bead and the rim bed, so called tire burping.

The tire contact patch of the wide rim is inside the rim flanges. The side wall of the tire is supported by the wide rim which increases control. A wider rim can therefore be ridden with lower air pressure than a narrower rim.

A wheel with low tire pressure will roll better over rough surfaces than a wheel with high pressure, as the tire can adapt better to the ground. A wheel with high air pressure cannot absorb obstacles and the bike must move upwards, comparable to lifting work, which increases rolling resistance. Less air pressure leads to less fatigue, because as a rider you also have less lifting work to do.

The most beautiful trails can often only be reached by your own muscle power. Long ascents and carrying passages require a light wheelset. On the descent, however, the wheel must be stable enough to withstand obstacles like roots and rocks. This is exactly what the XMC 1200 rim was developed for.

The low weight of the XMC 1200 SPLINE reduces the moment of inertia, allowing a higher speed to be achieved with the same energy input compared with aluminum wheels. The low moment of inertia saves energy and allows longer ascents.

The impact resistance lies between the EXC and XRC. 
The XMC 1200 is classified as ASTM Category 4 with a maximum system weight of 120 kg, which represents the use of components under the conditions in very rough, blocked terrain. Exactly what you need for your adventures.

DT Swiss rims are known for their impact resistance. We have high requirements for the impact resistance of an enduro rim. More than 20 different carbon architecture layups with different materials were tested until the impact strength met our high standards. The key factors for an impact-resistant rim are the arrangement of the individual carbon layers, the ideal ratio between resin and carbon fibers and meticulous production process control.
All these factors make the EXC 1200 SPLINE one of the most impact-resistant carbon enduro rims on the market.

Our internal tests showed that the measures were successful. We defined our impact resistance requirements based on experience with our aluminum rims. 
The comparison to our competitors with two of the most relevant carbon rim producer showed that we have higher demands for the impact strength of an Enduro rim. The DT Swiss EXC 1200 Spline was able to absorb 18% more energy without a critical failure. 

We conduct extensive product tests in our internal test center. In addition to the tests prescribed by the UCI, we are constantly developing new product tests. The aim is to reproduce as accurately as possible the damage patterns that occur in reality. We test impact resistance using a so-called "puncture test". The aim of this test is to investigate the local fracture behavior of the rim, which is known from the field, e.g. when you hit a stone or a root.